Popular design principles suggest that maximizing gap radius (\(r_g\) ) of a motor is the best way to address the challenge of motor selection for QDDs; this strategy is founded on analysis of ideal motor models, trends shown on a large range of motors [1]}, [2]}, [3]}, [4]}, and encouraging performance of modern robots with high-radius motors [5]}, [6]}, [7]}. However, for a specific application, the best motor choice may not have the highest \(r_g\) .
We propose a pair of simple selection metrics independent of \(r_g\)  to identify motors with useful properties for legged robots (Section ).
We also provide a case study of the RI50 motor, which defies the larger-scale trends (Section REF ).
Methods for characterizing the RI50 motor are described in Section ;
results and comparisons are summarized in Section .
A physics-based rationale to explain the observed differences between real motors and scaling-based predictions is proposed in Section .
